[npm] Preserve requirement source when updating transtive dep parents

[mctofu]

We had been setting the source of transitive parent dependencies to nil but this caused the FileUpdater to think the requirement was always changing. This might not be the case for a library with a requirement range that already allows the fixed version. We should prefer to preserve the existing source when an updated source is not provided.

Example:

#<Dependabot::Dependency:0x0000558c3bf7fc48
 @name="serverless-offline",
 @package_manager="npm_and_yarn",
 @previous_requirements=
  [{:requirement=>"8",
    :file=>"package.json",
    :groups=>["devDependencies"],
    :source=>{:type=>"registry", :url=>"https://registry.npmjs.org"}}],
 @previous_version="8.8.0",
 @removed=false,
 @requirements=[{:requirement=>"8", :file=>"package.json", :groups=>["devDependencies"], :source=>nil}],
 @version="8.8.1">

The version is updating from 8.8.0 to 8.8.1 but because this came from a library the requirement did not need to change. However, the source change between the previous/current requirements triggers an attempt to update package.json which results in this failure:

updater | ERROR <> Expected content to change!
updater | ERROR <> /home/dependabot/npm_and_yarn/lib/dependabot/npm_and_yarn/file_updater/package_json_updater.rb:36:in `block (2 levels) in updated_package_json_content'

The smoke test is failing on this PR because its expectations don't currently include the source populated in the new requirements.